The present invention relates to a transmitter used for a communication system having a high peak-to-average power ratio (PAPR).
Known examples of a system having a high PAPR include an orthogonal frequency division multiplexing (OFDM) communication system. FIG. 1 shows a typical OFDM transmitter. As shown in FIG. 1, the OFDM transmitter includes: a serial-to-parallel converter (S/P) 1 for performing serial-to-parallel conversion on an input bit stream (source of data to be transmitted); an inverse fast Fourier processor (IFFT) 2 having a size of M; a parallel-to-serial converter (P/S) 3; an oversampling block (interpolator) (OS) 4; a digital-to-analog converter (DAC) 5; an output low-pass filter (LPF) 6; an RF up converter (RF) 7; and a high-power amplifier (HPA) 8 that operates in class A or class A-B.
The OFDM transmitter shown in FIG. 1 generates an OFDM signal having a high PAPR level. In order to amplify the high-PAPR OFDM signal, it is necessary to employ a high input back-off level linear amplifier that operates in class A or class A-B. However, the amplifier of this type is low in power efficiency.
The simplest approach to reducing a PAPR of the OFDM signal is to clip (cut out) high amplitude peaks thereof. A variety of clipping techniques have been proposed. In some of the techniques, outputs of an inverse fast Fourier transform (IFFT) are clipped before interpolation (oversampling (OS)). However, the signal must be interpolated before digital-to-analog conversion, thus causing peak re-growth. To avoid the problem of peak re-growth, the signal may be clipped after interpolation. However, such the scheme causes extremely significant out-of-band power.
Examples of approaches for reducing the out-of-band power to a desired level include a PAPR reducing scheme for effecting such filtering as described below, which enables reduction of the out-of-band power to a desired level. FIG. 2 shows a configuration example of an OFDM transmitter (conventional example) to which the PAPR reducing scheme using a “clipping and filtering method” is applied.
The OFDM transmitter shown in FIG. 2 is provided with: an IFFT 9 in place of the IFFT 2 shown in FIG. 1; a limiter 10 to which an output (OFDM signal) from the P/S 3 is inputted; and an LPF 11 to which an output from a limiter 10 is inputted, the LPF 11 being connected to the DAC 5.
First, an input vector (input bit stream) is converted from a frequency domain to a time domain by using an oversize IFFT 9. In a case where an oversampling factor of the IFFT 9 is K, the input vector is extended by adding M×(K−1) zeros in the middle of the vector. As a result, trigonometric interpolation is performed in the time domain. The oversampled or interpolated signal is then clipped at the limiter 10. In the limiter 10, hardware-limiting is applied to the amplitude of the complex values x at the IFFT output.
                    [                  Expression          ⁢                                          ⁢          1                ]                                                                                  Limitter            ⁢                                                  ⁢            input            ⁢                          :                        ⁢                                                  ⁢                          S              ⁡                              (                t                )                                              =                      ρ            ·                          exp              ⁡                              (                                  j                  ·                  ϕ                                )                                                    ⁢                                  ⁢                              Limitter            ⁢                                                  ⁢            output            ⁢                          :                        ⁢                                                  ⁢                                          S                *                            ⁡                              (                x                )                                              =                      {                                                                                S                    ⁡                                          (                      t                      )                                                                                                                                  for                      ⁢                                                                                          ⁢                      ρ                                        <                    A                                                                                                                    A                    ·                                          exp                      ⁡                                              (                                                  j                          ·                          ϕ                                                )                                                                                                                                                        for                      ⁢                                                                                          ⁢                      ρ                                        >                    A                                                                        }                          ⁢                                  ⁢                              Clip            ⁡                          (              t              )                                =                                    S              ⁡                              (                t                )                                      -                                          S                *                            ⁡                              (                t                )                                                                        (        1        )            
In Expression 1, A is a clipping level or a clipping ratio (CR), Clip(t) is a clip signal (difference between an original signal and a signal as an output from the limiter 10), and S(t) is an (unclipped) OFDM signal. Note that both x and y are complex values.
The clipping ratio CR is defined as a ratio of the clipping level to mean (average) power of an unclipped baseband signal. After the clipping, the LPF 11 performs frequency-domain low-pass filtering to effect waveform shaping and reduction in out-of-band power.
FIG. 3 shows a cumulative distribution function (CCDF) of the OFDM transmitter shown in FIG. 2 in terms of different clipping levels (CR). In simulations shown in FIG. 3, a modulation scheme is quadrature phase shift keying (QPSK), where M=64 and K=2. A plotted (dotted) graph that corresponds to an OFDM transmission scheme (OFDM transmitter) shown in FIG. 1 is indicated by “OFDM” in FIG. 3. The filtering performed by the LPF 11 exhibits some peak re-growth (see FIG. 3). More compact CCDF is preferable in order to reduce a dynamic range of the HPA 8 and increase power efficiency of the HPA 8.
To remove the resulting out-of-band power, the clip signal is filtered in the LPF 11. In the conventional example, a filter unit is composed of a pair of larger FFT and IFFT modules. A filter task is based on a finite-impulse response (FIR) with 103 coefficient and recursive filters. The FIR is implemented by convolution summation.
                    [                  Expression          ⁢                                          ⁢          2                ]                                                                      y          ⁡                      (            n            )                          =                              ∑                          k              =              0                                      N              -              1                                ⁢                                    h              ⁡                              (                k                )                                      ·                          x              ⁡                              (                                  n                  -                  k                                )                                                                        (        2        )            
In Expression 2, y(n) is an FIR output signal, x(n) is an FIR input signal, h(n) is unit samples of the FIR, and N is a length of the FIR (number of filter taps).
FIG. 4 shows a structure of an FIR digital filter based on Expression 2. In order to implement Expression 2, the FIR digital filter includes: a plurality of delay units for obtaining sample values from the FIR input signal x(n); a plurality of multipliers for multiplying the sample values by predetermined constants (weighting factors h0, h1, . . . , hN-1); and a plurality of adders for obtaining the FIR output signal y(n) by summing up outputs from the plurality of multipliers. By control of the constants (weighting factors) given to the multipliers, the FIR digital filter may constitute an LPF or an HPF. The LPF 11 shown in FIG. 2 is formed of the FIR digital filter to remove out-of-band components from a main signal (to reduce out-of-band power).
As can be seen from FIG. 4, the number of multiplication operations for an FIR with N taps equals N. In an OFDM system using a large number of subcarriers, in order to obtain strong out-of-band power suppressions (typically, −50 to −80 dB), the total number of the weighting factors for the FIR must be large. For example, FIG. 5 shows a level of the out-of-band power suppressions (ATT) expressed in dB for a WiMAX system with 2048 subcarriers when clipping level equals 4 dB and the number of taps N of the FIR as a parameter.
In the plotted (dotted) graph of FIG. 5, as the length of the FIR (number of taps N) increases, the out-of-band power suppression further improves. Thus, according to the simulation results shown in FIG. 5, the FIR with 512 taps provides satisfactory out-of-band power suppression. On the other hand, it is obvious that FIRs with less than 512 taps provide unsatisfactory out-of-band power suppression.
In a case of implementing the FIR digital filter with an application specific integrated circuit (ASIC) implementation or a field programmable gate array (FPGA) implementation, it is necessary to provide a large number of 8-bit multipliers corresponding to the number of taps N. The 8-bit multiplier is about 8 times more complex than the adder. Thus, the conventional example has a problem in that the FIR digital filter becomes larger and more complex. There is another problem in that the FIR digital filter requires more time for a low-pass filtering process.
In addition to the OFDM transmitters shown in FIGS. 1 and 2, prior art with respect to the present invention includes techniques described in the following related art documents.
[Patent document 1] JP 2001-189630 A
[Patent document 2] JP 2002-368716 A